1. Field of the Invention
The present invention relates to a method of incinerating waste materials such as waste tires, for example, through dry distillation and gasification by thermally decomposing and gasifying a waste material by way of dry distillation to produce a combustible gas, burning the combustible gas, and imparting the heat of combustion to a heat source such as a boiler or the like.
2. Description of the Prior Art
One known process of incinerating waste materials such as waste tires, for example, is disclosed in Japanese laid-open patent publication No. 5-296427. According to the disclosed process, a waste material is placed in a fully closed gasification furnace and a portion of the waste material is burned while the remainder of the waste material is thermally decomposed through dry distillation with the heat of combustion. A combustible gas produced by the waste material when it is thermally decomposed is introduced from the gasification furnace into a combustion furnace positioned outside of the gasification furnace, and the introduced combustible gas is burned in the combustion furnace.
During the above incinerating process, the gasification furnace is supplied with oxygen (air) necessary to carry out the dry distillation through an oxygen supply passage, and the temperature at which the combustible gas is burned in the combustion furnace is detected. The opening of a valve disposed in the oxygen supply passage is controlled depending on the detected temperature through a feedback loop so that the detected temperature will be equalized to a certain substantially constant temperature.
In the incinerating process, the temperature at which the combustible gas is burned in the combustion furnace, i.e., the combustion temperature of the combustible gas, varies as indicated by the solid-line curve a in FIG. 2 of the accompanying drawings. Specifically, after a portion of the waste material in the gasification furnace has been ignited, starting dry distillation, the combustion temperature of the combustible gas in the combustion furnace gradually increases as the amount of a generated combustible gas gradually increases. When the combustion temperature of the combustible gas reaches a predetermined temperature, the opening of the valve in the oxygen supply passage is controlled to keep the combustion temperature continuously at the predetermined temperature. When the remainder of the waste material which can be thermally decomposed is reduced in amount as the dry distillation is in progress, since the amount of a generated combustible gas is reduced even though the opening of the valve is controlled, the combustion temperature of the combustible gas in the combustion furnace gradually decreases from the predetermined temperature.
According to the above incinerating process, the waste material can be incinerated in a manner to prevent gases such as NOx, etc. which are harmful to environments from being emitted when the predetermined temperature is set to a suitable temperature. Furthermore, the heat of combustion of the combustible gas can effectively be utilized when it is imparted to a heat source of an apparatus which utilizes heat energy, such as a boiler or the like. The apparatus such as a boiler or the like can be operated efficiently during a period of time in which the combustion temperature of the combustible gas is maintained at the substantially constant temperature.
It is desirable to process a large amount of waste material efficiently in the incinerating process. If the heat of combustion of the combustible gas in the combustion furnace is utilized as the heat source of the apparatus such as a boiler or the like, it is also preferable to be able to operate the apparatus efficiently over a continuous period of time. These demands may be met by increasing the capacity of the gasification and combustion furnaces for processing a large amount of waste material. However, a large amount of waste material placed in the gasification furnace generally prevents the heat from being smoothly propagated for dry distillation, making it difficult to thermally decompose the waste material stably and sufficiently through dry distillation. When this happens, the amount of a generated combustible gas is unstable, and the combustion temperature of the combustible gas in the combustion furnace is also unstable, tending to generate undesirable gases such as NOx, etc. and failing to stably operate the apparatus such as a boiler or the like efficiently.
In view of the above problems, the inventor attempted to provide two or more sets of gasification and combustion furnaces, and thermally decompose a waste material through dry distillation and burn a produced combustible gas successively or alternately at suitable times in the gasification and combustion furnaces of the sets, for thereby processing a large amount of waste material and imparting the heat of combustion of the combustible gas successively or alternately from the combustion furnaces to the heat source of an apparatus such as a boiler or the like, so that the apparatus can operate continuously over a long period of time.
More specifically, if there are two sets of gasification and combustion furnaces available, then a waste material is thermally decomposed through dry distillation and a generated combustible gas is burned in one of the two sets of gasification and combustion furnaces, and the heat of combustion of the combustible gas is imparted to the heat source of an apparatus such as a boiler or the like. When the dry distillation of the waste material in the gasification furnace is finished, a waste material starts being thermally decomposed through dry distillation and a generated combustible gas starts being burned in the other set of gasification and combustion furnaces, and the heat of combustion of the combustible gas is imparted to the heat source of the apparatus such as a boiler or the like.
The above combination of the sets of gasification and combustion furnaces makes it possible to process a larger amount of waste material and increase the period during which the head of combustion is imparted to the apparatus such as a boiler or the like, resulting in an increased period of time during which the apparatus can operate continuously.
When the waste material is incinerated and the apparatus such as a boiler or the like is operated using the two sets of gasification and combustion furnaces, the other next set of gasification and combustion furnaces should preferably begin to operate with such timing that when the combustion temperature (see the solid-line curve a in FIG. 2) of the combustible as produced by the operation of the first set of gasification and combustion furnaces starts being lowered from the predetermined temperature, the combustion temperature of the combustible gas produced by the operation of the next set of gasification and combustion furnaces rises to the predetermined temperature as indicated by the broken-line curve a.sub.2 in FIG. 2. With such starting of operation of the next or subsequent set of gasification and combustion furnaces, the temperature of the heat of combustion imparted to the apparatus such as a boiler or the like is kept at the predetermined temperature after the combustion temperature of the combustible gas in the combustion furnace of the prior set has reached the predetermined temperature until the combustion temperature of the combustible gas in the combustion furnace of the subsequent set begins to drop from the predetermined temperature. During this time, the amount of thermal energy applied to the apparatus such as a boiler or the like per unit time is kept substantially constant, so that the apparatus can stably be operated efficiently over a prolonged period of time.
If the time to operate the subsequent set of gasification and combustion furnaces is delayed from the above timing, i.e., if the combustion temperature of the combustible gas in the combustion furnace of the subsequent set reaches the predetermined temperature, as indicated by the imaginary-line curve a.sub.x in FIG. 2, after the combustion temperature of the combustible gas in the combustion furnace of the prior set has dropped a certain level from the predetermined temperature, then since the temperature of the heat of combustion imparted to the apparatus such as a boiler or the like temporarily falls greatly, the apparatus tends to operate unstably. If, on the other hand, the combustion temperature of the combustible gas in the combustion furnace of the subsequent set reaches the predetermined temperature, as indicated by the imaginary-line curve a.sub.y in FIG. 2, while the combustion temperature of the combustible gas in the combustion furnace of the prior set is being still maintained at the predetermined temperature, i.e., if the periods in which the combustion temperatures of the combustible gas in the combustion furnaces of the prior and subsequent sets are maintained at the predetermined temperature overlap each other, then the temperature of the heat of combustion imparted to the apparatus such as a boiler or the like is maintained, but the amount of thermal energy imparted to the apparatus per unit time is excessively large in the overlapping periods, with the result that the apparatus tends to operate unstably. In this case, the period of time in which the required heat of combustion is continuously imparted to the apparatus such as a boiler or the like is shorter than if the gasification and combustion furnaces of the subsequent set start to operate with appropriate timing as described above.
The inventor predicted that if the amount of a waste material accommodated in the gasification furnace (which corresponds to the capacity of the gasification furnace) remains the same, then the period of time (hereinafter referred to as a "temperature-constant time") in which the combustion temperature of the combustible gas is maintained at the predetermined temperature is substantially constant, and the period of time (hereinafter referred to as a "temperature-rising time") in which the combustion temperature of the combustible gas rises to the predetermined temperature after the gasification and combustion furnaces have started to operate is also substantially constant, and estimated and established a temperature-constant time and a temperature -rising time based on the above presumption. The inventor also attempted to start operating the gasification and combustion furnaces of the subsequent set upon elapse of a time that is calculated by subtracting the temperature-rising time from the temperature-constant time which has started from the time when the combustion temperature of the combustible gas in the combustion furnace of the prior set reached the predetermined temperature.
As a result of various research efforts, the inventor has found out that while the temperature-rising time is not largely affected by the manner in which the waste material is stored in the gasification furnace, the tempera- ture-constant time may not necessary remain constant even if the amount of waste material stored in the gasification furnace is the same, and is likely to vary relatively greatly depending on the manner in which the waste material is stored in the gasification furnace. Therefore, even if the gasification and combustion furnaces of the subsequent set start operating based on the temperature-constant time and the temperature-rising time as described above, the gasification and combustion furnaces may start operating too late or too early with respect to the appropriate timing. Consequently, it has been difficult to operate the gasification and combustion furnaces of the subsequent set reliably with suitable timing.